There has been proposed a digital audio disc which is a disc of relatively small diameter having a center hole and used as a record medium and on which digital information signals representing music or other are recorded in such a manner as to be read optically. The digital audio disc has a signal recording intermediate level which is formed in a plane portion extending around the center hole and on which information signals representing a number of units of music are recorded in such a manner as to be read by a player using a light beam for reading. One of outer surfaces of the digital audio disc facing each other with the signal recording intermediate level between is used as a signal reading surface.
When the information signal recorded in the digital audio disc is reproduced, a light beam is caused to impinge through the signal reading surface upon the signal recording intermediate level to read the information signal recorded thereon. Then, the light beam is reflected at the signal recording intermediate level to be formed into a relfex beam, and a reproduced information signal is obtained by detecting the relfex beam coming out the digital audio disc through the signal reading surface thereof. Accordingly, if the signal reading surface of the digital audio disc is damaged or wounded, the reproduced information signal is likely to include noise components resulting from the damage or wound on the signal reading surface so as to be deteriorated in its quality.
So, for the purpose of avoiding such a problem, the digital audio disc is usually contained in its entirety in a protecting case shaped into a box while it is not in use, so that the signal reading surface is protected against damage or wound.
As a disc with which reproduction of music sound is easily enjoyed, a kind of such a digital audio disc as aforementioned which is provided with a signal recording intermediate level on which a digital information signal representing only one or two units of music is recorded (hereinafter, referred to as a single disc)has been also proposed. The single disc is constituted, for example, as shown in FIG. 5 of the accompanying drawings.
In the single disc shown in FIG. 5, a signal recording intermediate level is formed in only a part of a plane portion 2 extending around a center hole 1, and a signal reading surface portion 3 is formed on only an annular portion in one of opposite surfaces of the plane portion 2, which is positioned to correspond to the signal recording intermediate level in the plane portion 2. An inner annular portion 4 in the surface of the plane portion 2, which is positioned at the inside of the signal reading surface portion 3 and an outer annular portion 5 in the surface of the plane portion 2, which is positioned at the outside of the signal reading surface portion 3 are not used as signal reading surface portions but used as, for example, areas on which labels are stuck.
As for such a single disc, it is also required to protect the signal reading surface portion 3 against damage or wound. Therefore, the single disc is preferably contained in its entirety in a box-shaped protecting case while it is not in use, in the same manner as an ordinary digital audio disc having one of opposite surfaces of its plane portion used in its entirety as a signal reading surface portion.
However, in the case where the single disc is contained in its entirety in the box-shaped protecting case for protecting the signal reading surface portion thereof against damage or wound, there is an unreasonableness that the box-shaped protecting case in which the single disc is contained in its entirety is used for protecting the signal reading surface portion of the single disc which is formed on only a partial area in one of the opposite surfaces of the plane portion of the single disc, which is relatively small compared with the whole opposite surfaces, so that portions of the single disc which are not required to be covered are also covered undesirably by the box-shaped protecting case without any benefit compensating for the disadvantage of augmentation in the external dimensions. Further, although main advantages of the single disc reside in cheapness and easy handling, the use of the box-shaped protecting case in which the single disc is contained in its entirety results in spoilage of such main advantages.